1. Field of the Invention
The present invention relates to a sheet feeder for use in a copying machine, a facsimile machine, a printer or the like, and to a retard roller for use in such a sheet feeder.
2. Description of the Art
In sheet feeders for use in a copying machine, a facsimile machine and a printer, sheets from a sheet tray are fed into a sheet separator by a pickup roller where the sheets are separated from each other and fed out on a one-by-one basis. The sheet separator of the sheet feeder is generally provided with a multiple-sheet-feeding prevention mechanism of an FRR (Feed and Reverse Roller) type or an FR (Friction Retard) type so that the sheets assuredly can be separated from each other for the one-by-one sheet feeding.
In a sheet feeder having a multiple-sheet-feeding prevention mechanism of the FRR type, as shown in FIG. 5, a pickup roller 3 feeds a sheet 1 from a sheet tray 2 into a sheet separator. In the sheet separator, a retard roller 5 is disposed in abutment against a sheet feeding roller 4 provided with a torque limiter 6 to prevent multiple sheet feeding.
When a single sheet 1 is fed into the sheet separator in the sheet feeder, a frictional torque exerted on the retard roller 5 by the sheet feeding roller 4 rotating in a normal direction is greater than a threshold of the torque limiter 6. Therefore, the retard roller 5 also rotates in a normal direction to feed out the sheet 1. When two or more sheets 1 are fed into the sheet separator, on the other hand, the retard roller 5 is not influenced by the friction of the sheet feeding roller 4, and thus the friction torque does not reach the threshold of the torque limiter 6. Therefore, the retard roller 5 stops rotating, or rotates in a reverse direction, to stop movement of excess sheets, and only the uppermost sheet 1 is fed out in contact with the sheet feeding roller 4.
Exemplary materials used in the pickup roller 3, the sheet feeding roller 4 and the retard roller 5 in the sheet feeder are rubber materials including olefin rubbers such as EPDM, natural rubbers, polynorbornene rubbers, urethane rubbers, and urethane foams. These rollers are generally produced by: (1) cutting or forming a cylindrical roller body from a solid rubber material or urethane foam, inserting a shaft into the cylindrical roller body, and grinding the surface of the roller body; or (2) molding a cylindrical roller body around a metal shaft in a mold, and grinding the surface of the roller body.
When a higher reliability is required for the sheet feeding in the sheet feeder, the multiple-sheet-feeding prevention mechanism of FRR type is generally utilized which employs a retard roller in the sheet separator. The retard roller typically has a ground roller surface for preventing a reduction in the friction coefficient thereof due to aging. The ground roller surface is less liable to suffer from adhesion of foreign matter and paper dust generated during the sheet feeding, and maintains a sufficient friction coefficient during use. Further, the retard roller having a ground roller surface relatively easily satisfies dimensional accuracy requirements.
However, a conventional retard roller requires a grinding process in the production thereof, and the costs of this grinding process account for a high percentage of the production costs, thereby preventing cost reduction. Therefore, attempts have been conventionally made to develop a roller which requires no grinding process in production thereof. One example of such a roller is a roller having a textured roller surface similar to the ground roller surface as proposed in Japanese Unexamined Patent Publication Nos. 5-221059 (1993) and No. 8-108591 (1996).
However, the textured surface of this conventional roller which is brought into contact with a paper sheet is like a mirror surface, so that paper dust generated during the sheet feeding does not easily slip away, but rather is liable to adhere on the surface of the roller. This adhered dust makes it difficult for the roller to stably maintain a sufficient friction coefficient during prolonged use. Further, the roller has a relatively large mirror surface area. Therefore, when the roller is used as the retard roller, the roller is liable to suffer from a so-called stick-slip phenomenon which causes noises (creaky noises) at a relatively low sheet feeding speed (150 mm/sec or lower).
In the production of the conventional textured surface roller (the non-ground surface roller), an interior surface of a mold for formation of the textured surface is subjected to a shot blasting process or a chemical etching process. However, the shot blasting process fails to produce deep undulations on the interior surface of the mold, and the chemical etching process merely produces relatively smooth undulations (pseudo-mirror surface) on the interior surface of the mold. Therefore, the conventional textured surface roller has a difficulty in providing a performance comparable to the ground surface roller. When the conventional textured surface roller is employed as the retard roller, the roller suffers from a strain which occurs when the roller body is press-fitted around a resin or metal shaft in the production thereof and, hence, this type of roller has difficulty in satisfying requirements for the precision of the outer diameter and concentricity thereof.
Since the conventional textured surface roller fails to offer a retard roller performance comparable to the ground surface roller, the ground surface roller is currently employed as the retard roller.
In view of the foregoing, it is an object of the present invention to provide a less expensive retard roller of non-ground surface type which is virtually free from adhesion of paper dust generated during sheet feeding so as to have a long-term friction coefficient sustain ability comparable to the ground surface roller, and to be free from creaky noises at a relatively low sheet feeding speed. It is another object of the invention to provide a retard roller which is free from the influence of a strain which occurs when a roller body thereof is press-fitted around a shaft in the production thereof and, hence, the roller has an outer diameter and concentricity of improved accuracy.
It is further another object of the present invention to provide a sheet feeder which includes the aforesaid retard roller, and which is excellent in durability and free from creaky noises.
In accordance with a first aspect of the present invention to achieve the aforesaid objects, there is provided a retard roller for use in a sheet feeder, which comprises a roller body and a shaft extending axially through the roller body, the roller body having a textured roller surface comprising island portions and a sea portion recessed from the island portions, the island portions and the sea portion each having fine projections.
On the roller surface of the roller body of the inventive retard roller, the ratio S1/S2 of the total area S1 of the island portions to the area S2 of the sea portion is preferably 0.25 to 0.55. The fine projections on the island portions and the sea portion preferably each have a height of 3 to 25 xcexcm. Further, the island portions on the roller surface preferably each have a height of not smaller than 10 xcexcm, and are preferably spaced from each other by a peak-to-peak distance of not greater than 1.0 mm.
In the inventive retard roller, the shaft has an outer diameter progressively decreasing from one end to the other end thereof. One end of the shaft serves as an insertion end when a roller body material having a uniform outer diameter and a uniform inner diameter was press-fitted around the shaft during production of the roller.
In accordance with a second aspect of the present invention, there is provided a sheet feeder employing the aforesaid retard roller. In the inventive sheet feeder, the retard roller stops rotating or rotates in a reverse direction when a plurality of sheets are fed thereto.